The present invention concerns a strainer assembly and its method of operation. The assembly and method are suitable for use in a wide range of separation operations, in particular, but not limited to, the straining of sea water for use in offshore oil and gas exploration and production operations.
The removal of solid material from water and aqueous streams is required in many processes and operations. One such example is the straining of sea water required, in order to remove solid material including mud, sand, gravel and plant and animal matter, enabling the water to be used in drilling and other operations associated with the exploration and production of oil and gas. Other applications for straining include the cleaning of water produced from subterranean wells, including oil and gas wells, the cleaning and purification of aquifer water, river water and estuary water.
Known strainer assemblies comprise one or more strainer screens extending across a fluid flow path in a vessel or conduit between an inlet for raw fluid and an outlet for strained fluid, the fluid being caused to pass through the screen. In a simple configuration, the strainer screen extends across a pipe or vessel through which the fluid being cleaned is passed. In order to increase the capacity of an assembly, it is known to arrange the strainer material into one or more baskets or candles. One or more such baskets may be arranged within a vessel, to provide a higher surface area of strainer screen per unit volume of the vessel or conduit.
As straining proceeds, solid material is retained on the screen. The accumulation of solid material causes an increase in the pressure drop across the screen, in turn reducing the volume flowrate of fluid through the strainer, leading to reduced straining efficiency. Systems and procedures for removing the solid build up on strainer screens are known and generally comprise reversing the flow of fluid, such that a fluid, often a portion of the strained fluid, is passed through the screen in the opposite direction, removing the solid material. Such a procedure is known in the art as ‘backwash’. The strainer system generally comprises a separate conduit and outlet for removing the backwash fluid and the entrained solid debris. In the simple strainer configuration mentioned hereinbefore, the backwash assembly is similarly simple and straightforward. However, when more complex strainer assemblies are employed, the backwash system increases in complexity. This is particularly the case when it is desired to conduct a backwash cycle on a portion of the strainer assembly, while still keeping the remainder of the system available for use.
In one known arrangement, each strainer basket or candle is located in its own conduit, having a fluid inlet and a fluid outlet. Several such conduits are connected by their respective inlets and outlets to a series of fluid headers. In normal operation, raw fluid is feed via a feed header to each strainer candle. Strained fluid is removed via an outlet header. By the operation of appropriate valves, each strainer candle may be isolated from the feed and outlet headers and connected instead to a backwash inlet header and drain header, by means of which a backwash fluid may be passed in reverse direction through the strainer candle to remove solid material and clean the strainer screen. It will be appreciated that such an arrangement is complex in terms of the pipework and valve arrangements required. In addition, the overall assembly occupies a relatively large volume; a disadvantage when the available space is at a premium, for example on a subsea wellhead installation or offshore production platform.
In an alternative known arrangement, a plurality of individual, generally cylindrical strainer candles are mounted in a single vessel. In one particular assembly, the strainer candles extend longitudinally within the vessel between two supporting plates, each plate having an opening in communication with the interior of each strainer candle. An inlet for raw fluid is provided in one end of the vessel, whereby the raw fluid is caused to flow through the respective support plate and into the bore of each candle. Strained fluid is collected from around the candles and leaves the vessel through an appropriate outlet. Solid particles in the raw fluid are retained on the inner surface of the strainer candles.
A backwash system is provided to clean the strainer candles and remove the accumulation of solid material. In its simplest form, the backwash system simply reverses the direction of fluid flow through the vessel and the strainer candles. However, in order for the backwash operation to be completed, the entire vessel must be taken off line.
An improved backwash assembly applies the backwash procedure to one or several strainer candles, while leaving the remaining candles in operation. The system comprises a back wash assembly having one or more rotatable backwash conduits in one end of the vessel. The back wash conduits are brought into cooperation with successive strainer candles and cause fluid to flow along the interior of the candle, removing the accumulated solids by means of a shearing fluid flow. This system is complex to construct and maintain. It relies upon a seal being formed between the backwash conduit and the end of the strainer candle and is prone to leaks and poor backwash efficiency when the seals fail. An improved strainer and backwash system is therefore required.
One such system is the METROL® SEA-SCREEN® Coarse Strainer, commercially available from Petreco International. The system comprises one or more cylindrical strainer baskets arranged within a generally cylindrical vessel. Each strainer basket is provided with a dedicated backwash assembly having an elongate backwash collector head extending the length of the respective strainer basket. The collector head comprises a slot or a plurality of slots adjacent the inner surface of the strainer basket and connected to a backwash conduit in the backwash assembly. In operation, the collector head is rotated within the strainer basket so as to cause the slot to pass across the inner surface of the basket. The backwash conduit is connected to an outlet at reduced pressure, typically by being vented to atmospheric pressure, causing fluid within the vessel to enter the collector head through the slot. Fluid is drawn both through the strainer basket in the reverse direction to normal flow in the region adjacent the collector head, as well as across the inner surface of the strainer basket. In this way, solids accumulated on the inner surface of the strainer basket are removed by a reverse and a shear fluid flow.
While the aforementioned system provides a high straining capability with a very high backwash efficiency, there is a need for still further improvements for some specific applications of the straining concepts. For example, the system requires a motor and gearbox assembly for each strainer basket. While this is generally acceptable, there are circumstances where the overall weight of the strainer unit needs to be reduced. It would also be an advantage if the surface area of straining material per unit volume of the containment vessel could also be increased. This would allow the diameter of the vessel to be decreased for a given straining duty, reducing the size of the unit and further reducing the overall weight of the vessel, both empty and when filled with fluid and in operation.